# 12V Sealed lead acid battery level sensing

I know that you can easily determine the percent capacity remaining in a battery using an arduino by dividing the voltage down to something that the ADC can read. What I'm stuck on is how the voltage will vary as the battery discharges. I've looked at multiple datasheets and tried to look on Duracell's website for one (I'm using a Duracell Ultra SLA battery), but I couldn't figure out how the voltage behaves as the battery discharges.

The main thing I need to figure out is the voltage at which the battery is considered "dead." But I think I also need to figure out how to read the voltage curves in the datasheets because (from what I can tell) the voltage does not vary linearly with the battery's capacity remaining. Can anyone point me in the right direction for information about this?

• If this will be used for a single application you could create a battery curve (or discharge profile) by using a fixed load and measuring the voltage regularly. You'll then have a rough idea of the expected voltage at each point. You could also use a coulomb counter and forget about voltage altogether. Jul 25, 2017 at 22:47
• @David The application will be a solar-powered project, so the battery will have a variable amount of charging and load at any given time, so I don't think I can use a coulomb counter. As for the battery curve, what kind of fixed load could I use for that? Would a motor work? Jul 25, 2017 at 22:51
• Anthony, be prepared that by measuring the battery voltage, you will be able to estimate the state of charge within about 25%, but not more accurately. Battery voltage is not an accurate method for estimating state of charge, although it's better than nothing. Jul 26, 2017 at 2:00
• @NickAlexeev Yeah I've heard that open circuit voltage is not the best method, but I'm not concerned about having a ton of accuracy. I just want some idea of how fast the battery is discharging and if the solar panel is keeping up. Jul 26, 2017 at 2:27